This book must have been a major inspirational source for developers of toolkits not based on Xt. Because after reading the book, you will know why Xt was a giant mistake that only served to introduce additional problems into GUI programming.
For some reason, the book does not focus on Motif, but on Xaw, a completely useless set of widgets hacked onto Xt as a proof of concept. True, there are some free software programs using Xaw, but that was because they were written before other alternatives existed, and they generally all suck...
The simple fact that no other usable set of widgets for Xt than Motif exists, should be enough to convince you that Xt failed to reduce the complexity any more than it managed to introduce simply by sitting in the way of Xlib. A geometry manager or event-handling mechanism is not hard to write, and modern toolkits have avoided Xt completely.
There exists little reason to read this book today, as the only reason to use Xt is to use Motif, and this book won't help you with that. Still, if you work extensively with Motif, this book may carry some value...